Abstract: Both vertebrate (Bufo Marinus and Xenopus) and invertebrate (Limulus) photoreceptors will be studied to elucidate the mechanisms of visual transduction. There are four main goals: 1. We have developed a dialysis method for controlling the cytoplasm of Limulus photoreceptors and now will apply this method to vertebrate rods. If this proves possible, the quantitative dependence of the light-activated conductance on cai2+ and the interrelationship of Cai2+ and cyclic nucleotides will be studied. Another approach will be to study the changes in transduction that occur when antibodies to specific photoreceptor proteins are introduced into the cytoplasm of rods. 2. We propose to measure single-channel current in Limulus photoreceptors after removal of the surrounding glial cells. 3. To begin the objective identification of the internal transmitter for excitation in Limulus we will attempt to show that extracts of illuminated photoreceptors contain a factor that produces a receptor potential when introduced into dark adapted photoreceptors. 4. We will study the kinetics of the receptor potential in Limulus and search for a model that accounts for the kinetics.